J. LEES & P.MALIN, UC SANTA BARBARA / SCIENCE PHOTO LIBRARY J. LEES & P.MALIN, UC SANTA BARBARA / SCIENCE PHOTO LIBRARY
Seismic tomogram of the San Andreas fault at Parkfield, central California, showing the locat- ion of microquakes (yellow & white dots). Each image shows a horizontal cross-section (tomogram), 25km by 20km, at different depths. Green & blue squares are seismometers. Red & blue areas are different types of rock structure. Microquakes relieve strain caused by the crustal plates moving on either side of the San Andreas fault. Two- thirds of the way down the fault is a small area next to a seismometer that seems unrelieved by microquakes. This is the expected epicentre of the earthquake predicted to occur before 1993.
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